Automated controls for lumber drying kiln

ABSTRACT

A kiln for drying lumber having at least a single fan for circulating air within the kiln whereby a pair of probes is placed on the surface of each of two boards at different locations in the kiln and the fans reversed whenever the difference in moisture content between the two boards exceeds a predetermined amount. According to a further aspect of the invention, a pair of probes are imbedded in another board in the kiln to detect the moisture content of the interior of the board and a suitable signal generated and applied to a control mechanism which also receives a coded information carrier having a schedule of wet and dry bulb temperatures for each of a plurality of interior moisture contents. The wet and dry bult temperatures are automatically varied by the control mechanism in accordance with the schedule and the detected interior moisture content.

United States Patent [191 [111 3,744,144 Wei July 10, 1973 AUTOMATED CONTROLS FOR LUMBER DRYING KILN FOREIGN PATENTS OR APPLICATIONS Australia... 34/30 Primary Examiner-William F. ODea Assistant Examiner-William C. Anderson Attorney-Cushman, Darby & Cushman [57] ABSTRACT A kiln for drying lumber having at least a single fan for circulating air within the kiln whereby a pair of probes is placed on the surface of each of two boards at different locations in the kiln and the fans reversed whenever the difference in moisture content between the two boards exceeds a predetermined amount. According to a further aspect of the invention, a pair of probes are imbedded in another board in the kiln to detect the moisture content of the interior of the board and a suitable signal generated and applied to a control mechanism which also receives a coded information carrier having a schedule of wet and dry bulb temperatures for each of a plurality of interior moisture contents. The wet and dry bult temperatures are automatically varied by the control mechanism in accordance with the schedule and the detected interior moisture content.

' 8 Claims, 3 Drawing Figures I I {3001665 I "wwwf INVENTOR M ATTORNEYS AUTOMATED CONTROLS FOR LUMBER DRYING KILN BRIEF DESCRIPTION OF THE PRIOR ART AND SUMMARY OF THE INVENTION The invention relates to a lumber drying kiln and to a method and apparatus for controlling the conditions within the kiln.

Most lumber must be dried in a kiln under carefully controlled conditions of temperature and humidity before it can be used. Different species and thickness of lumber require different schedules in order to dry the lumber as quickly as possible and at the same time prevent the checking, grain collapse and case hardening which results when the exteriors of the boards dry much more rapidly than the interiors.

Dry kilns in use today have an exterior dial which is in communication with wet and dry bulb temperature sensors within the kiln and which displays the two temperatures. The dry bulb temperature is of course related to temperature alone while the wet bulb temperature varies as both a function of temperature and humidity. Further, a sample board is placed in the kiln and periodically removed and its interior moisture content checked. The operator then checks a printed schedule to determine the wet and dry bulb temperatures associated with the moisture content of the sample, and adjusts the temperature and humidity accordingly.

This manual approach has a number of drawbacks. First, considerable time is required by the operator who must make periodic checks. Further, even when the sample board is checked often, considerable time is wasted between when the lumber is ready for a change in wet and dry bulb temperatures and when the operator checks the sample board and makes the change. Any mistakes in interpreting the schedule or following the wrong schedule can be disasterous, particularly if the lumber is a board such as walnut or mahogany.

A further problem in such kilns results from the use of forced draft fans to circulate air within the kiln at a high velocity flow. Where the air enters the lumber pile it is obviously dryer than where it leaves the pile. If uncompensated, this results in non-uniform drying of the lumber. This problem is conventionally solved by manually reversing the fans about every 6 hours so that the air circulates in the opposite direction.

As an alternative to manual reversal, a conventional time clock can be employed to accomplish the same' function automatically. In both approaches, reversal is irrespective of the difference in moisture content between boards at different locations in the kiln.

Reversing at fixed time intervals means that for each drying cycle the fans have been reversed several times more than they actually needed to be reversed. The fans thus wear out much quicker than if they had been reversed only when really necessary. Further, the unnecessary starting and stopping adversely affects the electric load factor. Since a sizable portion of the electric bill is predicated on the load factor, the fewer reversals, the smaller the load factor and, accordingly, the smaller the electrical bill. 7

According to one aspect of this invention, two sensors are provided at two different locations in a lumber drying kiln for detecting the moisture contents of the surface of boards at those locations. Such sensors may each include a pair of electrical contacts or probes attached at different spots on a board so that the resistance between the probes is a function of the moisture content of the boards. The probes are each connected to different coils of a relay so that when the moisture content of one of the boards exceeds the moisture content of the other one of the boards by a given amount, e.g., 1 percent, the relay changes from a first to a second or from the second to the first condition. This sensitivity is preferably subject to control. The relay in turn is connected to a suitable apparatus for reversing the fans whenever the relay changes condition. Thus, unnecessary fan reversals are avoided.

According to a further aspect of this invention, a further sensor is provided in a board in the kiln for detecting the moisture content of the interior of any one of the boards in the kiln. A control mechanism receives a coded information carrier, such as a punched card or magnetic ink card bearing a coded schedule of wet and dry bulb temperatures for each of a plurality of interior moisture contents. The coded information carrier also preferably shows the moisture content of the lumber to the operator at all times. The control mechanism automatically adjusts the heat and humidity in the kiln in accordance with the sensed interior moisture content and the coded schedule. This mechanism also automatically can shut off the kiln at any predetermined moisture content of lumber. No supervision by a trained operator is necessary.

Many other objects and purposes of the invention will become clear from the following detailed description of the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a cut-away view of a kiln and a schematic of the apparatus for reversing the fans and controlling the wet and dry bulb temperatures within the kiln.

FIG. 2 shows a detailed view of one set of probes attached to the surface of a board.

FIG. 3 shows a detailed view of another set of probes embedded in a board.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 which shows a conventional kiln 20 with a plurality of boards 22 conventionally stacked therein for drying. The kiln is shown broken apart for convenience in illustrating. The boards may be mahogany, walnut or any other type of lumber. A conventional reversible fan 24 circulates air at a high velocity within kiln 20 as shown. Any number of such fans can be provided as necessary. Steam heating coils 26 are disposed conventionally in kiln 20 for heating the circulating air to any desired temperature and conventional humidity sprayor 27 adds moisture as desired to keep the humidity in kiln 20 at any desired level. The moisture added by sprayor 27 is controlled by valve 29. Two vents 31 are conventionally mounted in the kiln roof as shown.

As mentioned above, the fan or fans are reversed whenever the moisture content of a board at one location, preferably near where the circulating air enters the lumber stack, exceeds or is less than, by a predetermined amount, e.g., 1 percent, the moisture content of a board at another location in the kiln, preferably near where the circulating air leaves the lumber stack. As shown in FIG. 2, a first pair of electrical surface contacts 28 and 30 are attached to a board 32 by a platic block 33 as shown and serially connected with a voltage source 34 and one coil of a relay 36 which controls two switches 38 and 40. Board 32 is located near where the circulating air enters the stack of boards. Similarly, a second pair of surface contacts 42 and 44 are attached to a board 46 via plastic block 47 and serially connected with source 34 and a second coil of relay 36. Board 47 is located near where the circulating air leaves the stack of boards. The resistance between each pair of probes is a function of the moisture content of the board to which it is attached so that the current flowing through each coil is a function of the moisture content of one of the boards.

The coils are arranged so that whenever the current flowing through one coil exceeds the current flowing through the other by an amount such that the difference in moisture content between the two boards is a given amount, e.g., 1 percent, relay 36 changes from a first condition in which switch 38 is closed and switch 40 open to a second condition in which switch 38 is opened and switch 40 is closed or vice versa. Switches 38 and 40 are connected to fan control 43 which reverses the fan each time relay 36 changes its condition and shifts the positions of switches 38 and 40. Fan control 43 preferably includes one or more heavy duty relays for reversing the fan motors.

As can be seen in FIG. 3, a pair of further probes 52 and 54 are also imbedded in a further board 56 via block 57 and probes 52 and 54 are pushed into the interior of board 56 to detect the moisture content of the interior of the board. Board 56 is chosen at random. As mentioned above, it is the interior moisture content of the boards which determines how fast the boards can be dried. Probes 52 and 54 are connected to sensor 58 which applies a suitable voltage to probes 52 and 54 and generates a suitable signal which is applied to control 60 to indicate the interior moisture content. Control 60 also receives an information card 62, which may be a punched plastic or paper card. Card 62 indicates the desired wet and dry bulb temperatures for each of a plurality of moisture contents. Alternately, carrier 62 may be a card having the information coded in magnetic ink. By using a card having the desired schedule in for example, graphical form, the transition between temperatures and humidity levels can be gradual rather than abrupt. Any other suitable mechanism can be employed for sensing the coded information.

Control 60 functions as a computer in comparing the sensed moisture content signal received from sensor 58 with the moisture contents program of carrier 62. Whenever the sensed moisture content equals a content on carrier 62, control 60 automatically applies suitable signals to heating coils 26 and valve 29 so as to adjust the wet and dry bulb temperatures within kiln 20 to the values specified by carrier 62 for that interior moisture content. Thus, if the lumber enters the kiln at 40 percent moisture, control 60 causes the dry and wet bult temperatures to be adjusted to the values associated with 40 percent moisture content while if it enters at 20% the wet and dry bulb temperatures are adjusted to the values associated with 20 percent moisture content. If desired, a pointer or window in the computer could automatically show Moisture Content figures at bottom of card at all times. Further, the actual wet and dry bulb temperatures are preferably displayed exterior to a kiln 20. In FIG. 1, conventional wet and dry bulb sensors and 72 are connected to conventional display devices 74 and 76 for this purpose.

The following table gives a suitable schedule for one lumber species and thickness.

Dry-Bulb Wet-Bulb Moisture Content Temperature TCIHPCHIIUI'L Of of Over 40% I20 I I5 35 to 40 I20 I I2 30 to 35 I20 106 30 to 25 l30 1 I0 25 to 20 I40 I I0 20 to 15 150 H0 15% to Dry I I30 Many changes and modifications in the above embodiment of the invention can of course be made without departing from the scope of the invention and accordingly that scope is intended to be limited only by the scope of the appended claims.

I claim:

I. In a kiln for drying lumber having at least a single fan for circulating air within said kiln the improvement comprising:

first means for sensing the moisture content of a board at a first location in said kiln, second means for sensing the moisture content of another board at another location in said kiln, and

means connected to said first and second means for reversing said fan when the moisture content sensed by one of said sensing means exceeds the moisture content sensed by the other of said sensing means by a predetermined amount.

2. In a kiln as in claim 1 the further improvement wherein said predetermined amount is 1 percent.

3. In a kiln as in claim 1 wherein each said sensing means includes a AC voltage source and first and second electrical probes each attached to a board surface and wherein said reversing means includes relay means having a first coil connected in series with the probes of said second sensing means so that said relay means has a first condition when the current through said first coil exceeds the current through said second coil by a given amount has a second condition when the current through said second coil exceeds the current through said first coil by said given amount.

4. A method of drying lumber in a kiln having at least a single fan for circulating air within the kiln comprising the steps of:

sensing the moisture content of a board at a first location in the kiln,

sensing the moisture content of a board at a second location in the kiln;

reversing the fan whenever'the moisture content of the board at the first location exceeds the moisture content of the board at the second location by more than a predetermined amount and reversing the fan whenever the moisture content of the board at the second location exceeds the moisture content of the board at the first location by more than said predetermined amount.

5. Apparatus for controlling the wet and dry bulb temperature of the interior of a kiln for drying lumber comprising:

means for sensing the moisture content of the interior of a board in said kiln, and

means connected to said sensing means for receiving a coded information carrier having indicia indicating dry and wet bulb temperatures for each of a plurality of moisture contents of the interior of a second means for sensing the moisture content of another board at another location in said kiln, and

further means for sensing the moisture content of the interior of a board in said kiln and means connected to said further sensing means for receiving a coded information carrier having indicia indicating dry and wet bulb temperatures for each of a plurality of moisture contents of the interior of a board and for changing said wet and dry bulb temperatures in accordance with the sensed moisture content and said indicia. 

1. In a kiln for drying lumber having at least a single fan for circulating air within said kiln the improvement comprising: first means for sensing the moisture content of a board at a first location in said kiln, second means for sensing the moisture content of another board at another location in said kiln, and means connected to said first and second means for reversing said fan when the moisture content sensed by one of said sensing means exceeds the moisture content sensed by the other of said sensing means by a predetermined amount.
 2. In a kiln as in claim 1 the further improvement wherein said predetermined amount is 1 percent.
 3. In a kiln as in claim 1 wherein each said sensing means includes a AC voltage source and first and second electrical probes each attached to a board surface and wherein said reversing means includes relay means having a first coil connected in series with the probes of said second sensing means so that said relay means has a first condition when the current through said first coil exceeds the current through said second coil by a given amount has a second condition when the current through said second coil exceeds the current through said first coil by said given amount.
 4. A method of drying lumber in a kiln having at least a single fan for circulating air within the kiln comprising the steps of: sensing the moisture content of a board at a first location in the kiln, sensing the moisture content of a board at a second location in the kiln, reversing the fan whenever the moisture content of the board at the first location exceeds the moisture content of the board at the second location by more than a predetermined amount and reversing the fan whenever the moisture content of the board at the second location exceeds the moisture content of the board at the first location by more than said predetermined amount.
 5. Apparatus for controlling the wet and dry bulb temperature of the interior of a kiln for drying lumber comprising: means for sensing the moisture content of the interior of a board in said kiln, and means connected to said sensing means for receiving a coded information carrier having indicia indicating dry and wet bulb temperatures for each of a plurality of moisture contents of the interior of a board and for changing said wet and dry bulb temperatures in accordance with the sensed moisture content and said indica.
 6. Apparatus as in claim 5 wherein said sensing means includes first and second electrical probes imbedded in the middle of said board.
 7. Apparatus as in claim 5 wherein said carrier is a punched card.
 8. Apparatus for controlling conditions in the interior of a kiln for drying lumber having at least a single fan comprising: first means for sensing the moisture content of a board at a first location in said kiln, second means for sensing the moisture content of another board at another location in said kiln, and further means for sensing the moisture content of the interior of a board in said kiln and means connected to said further sensing means for receiving a coded information carrier having indicia indicating dry and wet bulb temperatures for each of a plurality of moisture contents of the interior of a board and for changing said wet and dry bulb temperatures in accordance with the sensed moisture content and said indicia. 